The invention relates to electric machines including motors and generators.
In operation, alternating current (AC) is applied to armature windings of a motor to generate a rotating magnetic field. The rotating field is used to generate torque between the rotor and the stator causing the rotor to turn. In embodiments in which the armature (AC) windings are positioned on the stator, the rotating field rotates in space and pulls the rotor with it. If the rotating field is generated by a fixed frequency AC power source, it rotates at a fixed speed (units of RPM).
The rotating field can pull or xe2x80x9cdragxe2x80x9d a moveable member, whether rotor or stator, in different ways. In one approach, the dragged member may be a permanent magnet or an electromagnet powered by direct current (DC). Motors are wound with two types of poles in their rotating field, North and South. The permanent magnet, or electromagnet, and the rotating field due to the stator current lock together, north pole to south pole, and rotate together.
In another approach, the dragged member of an induction motor is a rotor winding in which the stator generated rotating field induces a current. This current reacts with the rotating field to produce torque. To induce a current, the motor""s rotor winding must rotate slower than the rotating field; the difference in speed is called xe2x80x9cslipxe2x80x9d. Slip represents the inability of the rotor to keep up with the moving rotating magnetic field generated by the stator.
The dragged member of an AC synchronous motor is its rotor, which has either a permanent magnet or an electro-magnet. The motor rotation is synchronous with the AC line frequency because the rotor is locked to the rotating magnetic field which, in turn, is synchronous with the line frequency. Synchronous motors with two poles typically operate at 3600 RPM with 60 Hz power. Slower motors have four poles at 0 degrees (N), 90 degrees (S), 180 degrees (N), 270 degrees (S). Such motors run at 1800 RPM, synchronous, with 60 Hz power.
The main difference between a synchronous motor and an induction motor is that the rotor of the synchronous motor travels at the same speed as the rotating magnetic field due to stator currents. This is possible because the magnetic field of the rotor is created by field coils or permanent magnets. The rotor either has permanent magnets or dc excited currents, which are forced to lock into a certain position when confronted with another magnetic field. Thus, when the motor is operating at synchronous speed, slip and speed variation as a function of varying load does not exist.
However, with a synchronous motor, slip can occur in at least two situations. In one situation, if the load on the motor gets too high, the rotor may fall out of synchronization. In another situation, slip occurs when the motor is brought up to synchronous speed. One approach for addressing the problem of slip during start-up is to use an adjustable speed drive (ASD) to control the speed of the motor until it reaches synchronous speed. However, in some applications the cost of the ASD can surpass the cost of the motor itself. Thus, the use of an ASD may be cost prohibitive.
The problem of xe2x80x9cslipxe2x80x9d is particularly a problem when the winding or coil is wound with superconducting materials. When a pole slips the flux reverses through the superconducting coil. A large voltage is induced across the coil over a very short time. Most conventional windings with smaller number of turns can withstand this sharp increase in voltage for short periods. However, coils wound using superconducting materials require a large number of turns because the operating current of the wire (or tape) used to form the coil is relatively low. In this case, the high voltage, even for short periods of time, can be detrimental to a superconducting winding. For example, the high voltage can damage the insulative layers that surround the superconducting wire coils and cause a short circuit inside the coil.
In a general aspect of the invention, a superconducting winding includes a pair of superconducting winding sections electrically connected at a node and bypass circuitry connected between the node and electric ground. The bypass circuitry allows current to flow when a voltage across the superconducting winding exceeds a predetermined threshold voltage.
In another aspect of the invention, the method of providing a superconducting coil including electrically connecting a pair of superconducting winding sections at a node and connecting bypass circuitry between the node and electrical ground to allow current flow when a voltage across the superconducting winding exceeds a predetermined threshold voltage.
In still another aspect of the invention, a rotor assembly includes a support member; and the superconducting winding described above.
Embodiments of these aspects of the invention may include one or more of the following features.
The bypass circuitry includes a switching device (e.g., such as a zener diode, varistor, spark gap devices) having an open position and a closed position. The switch is in the closed position to allow current flow when the voltage across the superconducting winding exceeds the predetermined threshold voltage. The bypass circuitry includes a resistive element for dissipating power flowing through the bypass circuitry, which may be cryogenically-cooled. Each superconducting winding section includes a high temperature superconductor and may be formed as a pancake coil.
Among other advantages, the bypass circuitry protects the superconducting winding from potential damage due to overvoltage. The bypass circuitry allows the thickness of the coil insulation to be reduced, thereby providing additional space for superconductor. The bypass circuitry also provides a lower cost, simpler, and generally reliable protection mechanism for the superconducting winding.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.